This project is a study of the kinetics of energy transformation in active muscle using mechanical and myothermal measurements. Gilbert & Ford (1988) found that heat is absorbed during rapid tension recovery after quick release, indicating that the entropy change of the crossbridge 'power stroke' is positive. The detached-bridge transition in which the associated protein configurational change is reset in preparation for the next cycle must therefore have a negative entropy change and produce heat. One goal of the project is to quantify such thermodynamic parameters of crossbridge transitions, by measuring precisely the kinetics of heat changes during tension transients. Ford & Gilbert (1987) reported that a substantial amount of the heat produced specifically by shortening appeared afterwards, indicating its possible association with detached bridges and the 'power-stroke- reset'process. Another goal is to compare the post-shortening heat with the extents of shortening and the 'power-stroke' and 'reset' processes. Ford & Gilbert also showed that heart rate continues to rise after power becomes constant, indicating that the crossbridge cycle does not always reach a steady state. A third goal is to determine whether a steady state is established at all velocities and the kinetics with which it occurs. Sources of the rising heat rate are the negative entropy change of 'power-stroke- reset' and free energy dissipation by uncoupled bridges operating in the myosin cycle. Contributions from the two sources will be distinguished by the kinetics of heat production during the tran- sition to shortening steady state and from shortening to a new isometric steady state. The velocity dependence of energy coupling will also be determined. In the steady state net entropy changes of the contractile proteins must be zero, and most of the heat comes from dissipation of free energy. The ratio of power to total energy rate is therefore a measure of coupling. The observations will be used to develop a kinetic scheme of energy transduction with sufficiently specific energetic and thermodynamic properties to provide insight into the molecular mechanism. Motility and chemomechanical energy transduction are fundamental properties of living organisms, and variations of the two-protein, oarlike molecular mechanism operating in skeletal muscle are known to produce some of the other motile behavior at the subcellular level. Disruptions of cell motility produce many pathological conditions, whose etiology can be fully understood only if the underlying mole- cular mechanisms are known.